CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

14. The Formation of Species and Evolutionary Change

14.7. Human Evolution

There is intense curiosity about how our species (Homo sapiens) came to be, and the evolution of the human species remains an interesting and hot topic. Human beings are classified as mammals belonging to a group known as primates. Primates are thought to have come into existence approximately 66 million years ago. They include animals with enlarged, complex brains; five digits, with nails, on the hands and feet; and hands and feet adapted for grasping. Their bodies, except those of humans, are covered with hair. There are two groups of primates, the prosimians—lemurs and tarsiers—and the anthropoids—monkeys, apes, and humans (figure 14.12).

FIGURE 14.12. Where We Are on the Primate Tree

As new evidence is uncovered, scientists reconsider how that information best fits into the hypothesis on the evolution of humans. As the pieces of the puzzle are reexamined and rearranged, we see a clearer picture of where human beings fit in the scheme of life.

You cannot shake hands with any other species belonging to the genus Homo. All other versions of our close evolutionary relatives are extinct. This makes it difficult to visualize our evolutionary development. Therefore we tend to think we are not subject to the laws of nature. However, humans show genetic diversity, experience mutations, and are subject to the same evolutionary forces as other organisms.

Scientists use several kinds of evidence to try to sort out our evolutionary history. Fossils of various kinds of prehuman and ancient human ancestors have been found, but many of these are only fragments of skeletons, which are difficult to interpret and are hard to date. Stone tools of various kinds have also been found that are associated with prehuman and early human sites. Finally, other aspects of the culture of our ancestors have been found in burial sites, including cave paintings and ceremonial objects. Various methods have been used to date these findings. When fossils are examined, anthropologists can identify differences in the structures of bones that are consistent with changes in species. Based on the amount of change they see and the ages of the fossils, scientists make judgments about the species to which the fossil belongs.

As new discoveries are made, experts’ opinions will change, and our evolutionary history may become clearer as old ideas are replaced. Scientists must also review and make changes in the terminology they use to refer to our ancestors. The term hominin now refers to humans and their humanlike ancestors, whereas previously the term hominid was used. The term hominid now refers to the broader group that includes all humanlike organisms plus the great apes—gorillas, orangutans, chimpanzees, and bonobos. When you read material about this topic, you will need to determine how the terms are being used. Although there is no clear picture of how humans evolved, the fossil record shows that humans are a relatively recent addition to the forms of life. Members of the genus Homo are believed to have evolved at least 2.2 million to 2.5 million years ago (Table 14.1).

1. There is a great deal of fossil evidence that several species of the genera Australopithecus and Paranthropus were common in Africa beginning at about 4 million years ago. These organisms walked upright and are often referred to collectively as australopiths.

2. Based on fossil evidence, it appears that the climate of Africa was becoming drier during much of the time during which the evolution of humans was occurring.

3. The earliest Australopithecus fossils are from about 4.2 million years ago. Earlier fossils, such as Ardipithecus (which could have included a number of species), Orrorin, and Sahelanthropus may be ancestral to the genus Australopithecus. Ardipithecus would most likely be a “distant cousin” to human beings rather than a direct ancestor. Australopithecus and Paranthropus were herbivores and walked upright. Their fossils and those of earlier organisms, such as Ardipithecus, Orrorin, and Sahelanthropus, are found only in Africa.

4. The australopiths were sexually dimorphic, with the males much larger than the females, and they had relatively small brains (a cranial capacity of 530 cm3 or less—about the size of a standard softball).

5. Several species of the genus Homo became prominent in Africa beginning at about 2.2 million years ago and appear to have made a change from a primarily herbivorous diet to a carnivorous or omnivorous diet.

6. All members of the genus Homo have relatively large brains compared with the australopiths. The cranial capacity ranges from about 650 cm3 for early fossils of Homo to about 1,450 cm3 for modern humans. These organisms are also associated with various degrees of stone tool construction and use. Some of the australopiths may also have constructed stone tools.

7. The fossils of australopiths, several early species of Homo, and the fossils of earlier organisms such as Ardipithecus, Orrorin, and Sahelanthropus are found only in Africa. Fossils of several later species of the genus Homo have larger brains and are found in Africa, Europe, and Asia but not in Australia or the Americas. Only Homo sapiens is found in Australia and the Americas.

8. Since the fossils of Homo species found in Asia and Europe are generally younger than the early Homo species found in Africa, it is assumed that species of Homo migrated from Africa to Europe and Asia from Africa.

9. Differences in size between males and females are less prominent in the members of the genus Homo, so perhaps there were fewer sexual differences in activities than with the australopiths.

When scientists put all these bits of information together, they constructed the following scenario for the likely evolution of our species. Rather than humans evolving from a chimpanzee-like ancestor, chimps and humans evolved from a common primate ancestor. Early primates, ancestors to modern monkeys, chimpanzees, and apes, were adapted to living in forested areas, where their grasping hands, opposable thumbs, big toes, and wide range of shoulder movement, allow them to move freely in the trees. As the climate became drier, grasslands replaced the forests. Early hominins were adapted to drier conditions. Walking upright was probably an adaptation to these conditions. Most later hominins had large brains and used tools. A recent find, however, illustrates just how tentative the understanding of human evolution really is.

The discovery of fossils of a small human—known as the “hobbit”—in the sediments of a cave on an Indonesian island in 2003 has led to much speculation about a possible new species of Homo; Homo floresiensis. The fossils were dated to about 18,000 years ago. There has been much discussion among scientists about the significance of this discovery. Some feel that the small size indicates a developmental abnormality and that the remains could be that of an abnormally small H. sapiens. Others suggest that there are many examples of island animals that show small size and that these fossils were adapted to island conditions. Research continues and a more complete understanding will develop as more information is gathered. Like other human relatives, H. floresiensis is extinct. Therefore, the sole surviving member of our evolutionary line is Homo sapiens. We are now going to look at several organisms important to understanding the evolution of human beings.

HOW SCIENCE WORKS 14.1

Accumulating Evidence for Evolution

The theory of evolution has become the major unifying theory of the biological sciences. Those in medicine understand the dangers of mutations, the similarity in function of the same organ in related species, and the way in which the environment can interfere with the preprogrammed process of embryological development. Agricultural science has demonstrated the importance of selecting specific genetically determined characteristics for passage into new varieties of crop plants and animals. The concepts of mutation, selection, and evolution are so fundamental to an understanding of what happens in biology that we often forget to take note of the many kinds of observations that support the theory of evolution. The following list describes some of the more important pieces of evidence that support the idea that evolution has been a major force in shaping the nature of living things.

Fossil of sequoia twig

1. Species and populations are not genetically fixed. Change occurs in individuals and populations.

a. Mutations cause slight changes in the genetic makeup of an individual organism.

b. Different populations of the same species show adaptations suitable for their local conditions.

c. Changes in the characteristics displayed by species can be linked to environmental changes.

d. The selective breeding of domesticated plants and animals indicates that the shape, color, behavior, metabolism, and many other characteristics of organisms can be selected for.

e. The extinction of poorly adapted species is common.

2. All evidence suggests that, once embarked on a particular evolutionary road, structures, behaviors, or physiological processes are not abandoned, only modified. New organisms are formed by the modification of ancestral species, not by major changes. The following list supports the concept that evolution proceeds by the modification of previously existing structures and processes, rather than by catastrophic change.

a. All species use the same DNA code.

b. All species use the same left-handed (levorotary) amino acid building blocks.

c. It is difficult to eliminate a body part when it is part of a developmental process controlled by genes. A vestigial structure is an organ that is functionless and generally reduced in size. It resembles the fully functioning organs found in related organisms. Structures such as the human appendix, whale pelvic bones, and human tailbone are vestigial structures and evidence of genetic material from previous stages in evolution.

d. The embryological development of related animals is similar, regardless of the peculiarities of adult anatomy. All vertebrate embryos have an early stage that contains gill-like structures.

e. Species of organisms that are known to be closely related show greater similarity in their DNA than do those that are distantly related.

3. Several aspects of the fossil record support the concept of evolution.

a. The nature of the Earth has changed significantly over time.

b. The fossil record shows vast changes in the kinds of organisms present on Earth. New species appear and most go extinct. This is evidence that living things change in response to changes in their environment.

c. The fossils found in old rocks do not reappear in younger rocks. Once an organism goes extinct, it does not reappear, but new organisms arise that are modifications of previous organisms.

Archaeopteryx, fossil skeleton with feathers

4. New techniques and discoveries invariably support the theory of evolution.

a. The recognition that the Earth was formed billions of years ago supports the slow development of new kinds of organisms.

b. The recognition that the continents of the Earth have separated helps explain why organisms on Australia are so different from those elsewhere.

c. The discovery of DNA and how it works helps explain mutation and demonstrates the genetic similarity of closely related species.

d. Similarities in protein structure can indicate the degree of relatedness among organisms. For example, the greater the evolutionary distance from humans, the greater is the number of amino acid differences in the vertebrate hemoglobin polypeptide.

The Genus Ardipithecus

Ardipithecus ramidus lived in woodlands and was able to walk upright when on the ground as well as navigate on branches of trees. The fossils from the best-studied site have been aged at about 4.4 million years ago. Studies of these fossils indicate that Ardipithecus ramidus was about 120 cm (4 ft) tall when standing, weighed about 50 kg (110 lb), and had a brain capacity of less than 400 cm3. It was able to walk and run on its hindlimbs but had an opposable big toe and was able to climb and walk on the tops of branches on all fours. Males and females were about the same size. They ate a variety of plant and animal materials in their woodland habitat.

The Genera Australopithecus and Paranthropus

Both Australopithecus and Paranthropus are important evolutionary links in understanding our evolutionary past. Various species of Australopithecus and Paranthropus were present in Africa from about 4.4 million years ago until about 1 million years ago. Various members of these two extinct genera are often referred to as australopiths. Australopithecus is the genus to which the famed 3.2 million-year-old “Lucy” belongs, together with a 3.3 million-year-old A. afarensis, dubbed “Lucy’s baby.” Lucy’s baby, Selam, is estimated to have been about 3 years of age when she died and is the most complete hominin skeleton ever found. There are few fossils of these early humanlike organisms, however, and many fossils are fragments of organisms. This has led to much speculation and argument among experts about the specific position each fossil has in the evolutionary history of humans. However, from examinations of the fossil bones of the leg, pelvis, and foot, it is apparent that the australopiths were relatively short (males, 1.5 meters or less; females, about 1.1 meters) and stocky and walked upright, as humans do. They had relatively small brains (cranial capacity 530 cm3 or less—about the size of a standard softball).

An upright posture had several advantages in a world that was becoming drier. It allowed for more rapid movement over long distances and the ability to see longer distances, and it reduced the amount of heat gained from the sun. In addition, upright posture freed the arms for other uses, such as carrying and manipulating objects and using tools. The various species of Australopithecus and Paranthropus shared these characteristics and, based on the structure of their skulls, jaws, and teeth, appear to have been herbivores with relatively small brains.

The Genus Homo

About 2.5 million years ago, the first members of the genus Homo appeared on the scene. There is considerable disagreement about how many species there were, but Homo habilis is one of the earliest. H. habilis had a larger brain (650 cm3) and smaller teeth than the australopiths and made much more use of stone tools. Some experts feel that it was a direct descendant of Australopithecus africanus. Many experts feel that H. habilis persisted until about 1.44 million years ago. H. habilis made use of group activities, tools, and higher intelligence to take over the kills made by other carnivores. The higher-quality diet would have supported the metabolic needs of the larger brain.

About 1.8 million years ago, Homo ergaster appeared. It was much larger, up to 1.6 meters tall, than H. habilis, which was about 1.3 meters tall and had a much larger brain (a cranial capacity of 850 cm3). A little later, a similar species (Homo erectus) appeared in the fossil record and for awhile, coexisted with H. habilis. Some consider H. ergaster and H. erectus to be variations of the same species. Along with their larger brains and body size, H. ergaster and H. erectus are distinguished from earlier species by their extensive use of stone tools. Hand axes were manufactured and used to cut the flesh of prey and crush the bones to obtain the fatty marrow. These organisms appear to have been predators, whereas H. habilis was a scavenger. The use of meat as food allows animals to move about more freely, because appropriate food is available almost everywhere. By contrast, herbivores are often confined to places that have foods appropriate to their use: fruits for fruit eaters, grass for grazers, forests for browsers, and so on. In fact, fossils of H. erectus have been found in the Middle East and Asia, as well as Africa. Most experts feel that H. erectus originated in Africa and migrated through the Middle East to Asia.

At about 800,000 years ago, another hominin, classified as Homo heidelbergensis, appeared in the fossil record. Since fossils of this species are found in Africa, Europe, and Asia, it appears that they constitute a second wave of migration of early Homo from Africa to other parts of the world. Both H. erectus and H. heidel-bergensis disappeared from the fossil record as two new species (Homo neanderthalensis and Homo sapiens) become common.

The Neandertals were primarily found in Europe and adjoining parts of Asia and were not found in Africa. Since Neandertals were common in Europe, many people feel Neandertals are descendants of H. heidelbergensis, which also was common in Europe and preceded Neandertals (How Science Works 14.2).

FIGURE 14.13. Out-of Africa Hypothesis

Most scientists favor this explanation on the origin and dispersal of Homo sapiens that arose in Africa about 200,000 years ago. The first to leave ventured out some 70,000 to 50,000 years ago, reaching Asia and Australia about 50,000 years ago. Speculation is that they moved into the Americas about 20,000 to 15,000 years ago taking advantage of low sea levels and a land bridge that connect Siberia to Alaska.

Two Points of View on the Origin of Homo sapiens

Homo sapiens is found throughout the world and is now the only species remaining of a long line of ancestors. Two theories seek to explain the origin of Homo sapiens. One theory, known as the out-of-Africa hypothesis, states that modern humans (Homo sapiens) originated in Africa, as did several other similar species (figure 14.13). They migrated from Africa to Asia and Europe and displaced species such as H. erectus and H. heidelbergensis, which had previously migrated into these areas. The other theory, known as the multiregional hypothesis, states that H. erectus evolved into H. sapiens. During a period of about 1.7 million years, fossils of Homo erectus showed a progressive increase in the size of the cranial capacity and reduction in the size of the jaw, so it is difficult to distinguish H. erectus from H. heidelbergensis and H. heidelbergensis from H. sapiens. Proponents of this hypothesis believe that H. heidelbergensis is not a distinct species but, rather, an intermediate between the earlier H. erectus and H. sapiens. According to this theory, various subgroups of H. erectus existed throughout Africa, Asia, and Europe and interbreeding among the various groups gave rise to the various races of humans we see today.

Another continuing puzzle is the relationship of Homo sapiens to the Neandertals. Some people consider the Neandertals to be a subgroup of Homo sapiens specially adapted to life in the harsh conditions found in postglacial Europe. Others consider them to be a separate species, Homo neanderthalensis. The Neandertals were muscular, had a larger brain capacity than modern humans, and had many elements of culture, including burials. The cause of their disappearance from the fossil record at about 25,000 years ago remains a mystery. Perhaps a change to a warmer climate was responsible. Perhaps contact with Homo sapiens resulted in their elimination either through hostile interactions or interbreeding with H. sapiens, resulting in their absorption into the larger H. sapiens population.

Large numbers of fossils of prehistoric humans have been found in all parts of the world. Many of these show evidence of a collective group memory, called culture. Cave paintings, carvings in wood and bone, tools of various kinds, and burials are examples. These are also evidence of a capacity to think and invent, as well as “free time” to devote to things other than gathering food and other necessities of life. We may never know how we came to be, but we will always be curious and will continue to search and speculate about our beginnings. Figure 14.14 summarizes the current knowledge of the historical record of humans and their relatives.

FIGURE 14.14. Human Evolution

This diagram shows the various organisms thought to be relatives of humans. The bars represent approximate times the species are thought to have existed. Notice that (1) all species are extinct today except for modern humans, (2) several species of organisms coexisted for extensive periods, (3) all the older species are found only in Africa, and (4) more recent species of Homo are found in Europe and Asia, as well as Africa.

HOW SCIENCE WORKS 14.2

Neandertals—Homo neanderthalensis or Homo sapiens??

An ongoing controversy surrounds the relationship between the Neandertals and other forms of prehistoric humans. One position is that the Neandertals were a small, separate race or subspecies of human that lived in Europe and western Asia from more than 350,000 years ago to about 30,000 years ago. They could have interbred with other humans and may have disappeared because their subspecies was eliminated by interbreeding with more populous, more successful groups. (Many small, remote tribes have been lost as distinct cultural and genetic entities in the same way in recent history.) Others maintain that the Neandertals showed such great difference from other early humans that they must have been a different species and became extinct because they could not compete with more successful Homo sapiens immigrants from Africa. (The names of these ancient people typically are derived from the place where the fossils were first discovered. For example, the Neandertals were first found in the Neander Valley of Germany, and the Cro-Magnons, considered to be modern Homo sapiens, were initially found in the Cro-Magnon caves in France.)

The use of molecular genetic technology has shed some light on the relationship of the Neandertals to other kinds of humans. Examination of the mitochondrial DNA obtained from the bones of a Neandertal individual reveals that there are significant differences between the Neandertals and other kinds of early humans. This greatly strengthens the argument that the Neandertals were a separate species, Homo neanderthalensis.

In 2006, U.S. and German scientists began a two-year project to decipher Neandertals' genetic code. They used samples from a 38,000-year-old Neandertal fossil. They filtered out non-Neandertal DNA that had contaminated the samples following the death of the Neandertal individual. The hope is that this investigation will reveal the genetic differences in cognitive abilities (the process of being aware, knowing, thinking, learning, and judging) between Homo neanderthalensis and H. sapiens. To date, they have evidence suggesting that Neandertals and modern humans may have interbred, most likely H. sapiens fathering children with H. neanderthalensis females. Using the latest biotech techniques, scientists have found Neandertal and human genomes are between 99.5% and 99.9% identical. Some researchers believe that the most recent common ancestor of the two human species (H. sapiens and H. neanderthalensis) lived about 800,000 years ago. Others believe a more recent divergence time, about 465,000 to 569,000 years ago.

14.7. CONCEPT REVIEW

15. List three differences between australopiths and members of the genus Homo.

16. Compare the out-of-Africa hypothesis with the multiregional hypothesis of the origin of Homo sapiens.

17. Diagram the relationship among anthropoids, hominoids, and hominids.

Summary

Populations are usually genetically diverse. Mutations, meiosis, and sexual reproduction tend to introduce genetic diversity into a population. Organisms with wide geographic distribution often show different gene frequencies in different parts of their range. A species is a group of organisms that can interbreed to produce fertile offspring. The process of speciation usually involves the geographic separation of the species into two or more isolated populations. While they are separated, natural selection operates and each population adapts to its environment. If this generates enough change, the two populations may become so different that they cannot interbreed. Similar organisms that have recently evolved into separate species normally have genetic (reproductive) isolating mechanisms to prevent interbreeding. Some of these are habitat preference, seasonal isolating mechanisms, and behavioral isolating mechanisms. Many plants and some animals have a special way of generating new species by increasing their chromosome numbers as a result of abnormal mitosis or meiosis. Organisms that have multiple sets of chromosomes are called polyploids.

Evolution is basically a divergent process on which other patterns can be superimposed. Adaptive radiation is a very rapid divergent evolution; convergent evolution involves the development of superficial similarities among widely different organisms. The rate at which evolution has occurred probably varies. The fossil record shows periods of rapid change interspersed with periods of little change. This has caused some to look for mechanisms that could cause the sudden appearance of large numbers of new species in the fossil record, which challenge the traditional idea of slow, steady change accumulating enough differences to cause a new species to be formed. The early evolution of humans has been difficult to piece together because of the fragmentary evidence. Beginning about 4.4 million years ago, the earliest forms of Australopithecus and Paranthropus showed upright posture and other humanlike characteristics. The structure of the jaw and teeth indicates that the various kinds of australopiths were herbivores. Homo habilis had a larger brain and appears to have been a scavenger. Several other species of the genus Homo arose in Africa. These forms appear to have been carnivores. Some of these migrated to Europe and Asia. The origin of Homo sapiens is in dispute. It may have arisen in Africa and migrated throughout the world or evolved from earlier ancestors found throughout Africa, Asia, and Europe.

Basic Review

1. _____ _____ is the movement of genes from one generation to the next as a result of reproduction or from one region to another by migration.

2. A(n) _____ is any remains of an organism of a past geologic age, such as a preserved skeleton or body imprint.

3. Which of the following steps are not necessary for speciation to occur?

a. geographic isolation

b. genetic divergence

c. reproductive isolation

d. hybrid viability

4. The _____ of an organism is the geographic area over which a species can be found.

a. range

b. region

c. pasture

d. geographic location

5. If individuals from separate populations overcome the geographic barrier, they may not have accumulated enough _____ to prevent reproductive success.

a. mutations

b. genetic differences

c. barriers

d. sexual differences

6. _____ is a condition of having multiple sets of chromosomes, rather than the normal haploid or diploid number.

7. Differences in the time of the year at which reproduction takes place are called

a. geographic isolating mechanisms.

b. hybrid isolating mechanisms.

c. seasonal isolating mechanisms.

d. physical isolating mechanisms.

8. A _____ is a group of organisms that shares a common ancestor with other species, but is set off from those others by having newer, genetically unique traits.

9. The term _____ is now used to refer to humans and their humanlike ancestors.

a. hominid

b. anthropoid

c. hominoid

d. hominin

10. The scientific name for modern human beings is

a. Homo habilis.

b. Homo neanderthalensis.

c. H. erectus.

d. Homo sapiens.

11. Fossil of which small human found in Indonesia are speculated to be a new species of Homo?

a. the “hobbit”

b. Ida

c. Australopithecus sp.

d. Paranthropus sp.

12. Genetic _____ is a change in the allele frequencies of the isolated subpopulation compared to the rest of the species.

13. The _____ _____ concept is a companion hypothesis to gradualism and suggests a different way of achieving evolutionary change.

14. Which factor will not increase the likelihood that an organism is found in the fossil record?

a. The soft body parts decompose.

b. Marine organisms can be covered by sediment on the bottom.

c. Fossils of more recent organisms are less likely to have been destroyed by geological forces.

d. Fossils of large organisms are easier to find.

15. _____ structures are similar structures in different species that have been derived from a common ancestor.